Recent fires in copper solvent extraction plants have underscored the need for copper solvent extraction reagent formulations that would also provide increased electrical conductivity to the organic phase. See: “Static Electricity—A Danger Inherent in Solvent Extraction Processes”, G. Hearn and P. Smithson, Proceedings of the Alta Conference 2005; and “Electrostatic safety during the solvent extraction of copper”, J. N. Chubb, P. Lagos and J. Lienlaf, Journal of Electrostatics 63 (2005), pp. 119-127. As a result of such studies, it has generally been determined empirically that, in order to prevent static buildup in metal extraction circuits, a conductivity of about 250-500 pS/m, as determined by the methods of BS5958 Part I (British Standard Code of Practice for Control of Undesirable Static Electricity) is desirable in the organic phase in the solvent extraction circuit, depending on the velocity at which the organic phase is being pumped.
Since the organic phase in an extraction circuit is, generally, predominantly a kerosene mixture, with an inherently low conductivity, static electricity generated, e.g., by the organic solution flowing through non-conductive pipes or flexible hoses, circulating through partly-filled tanks, and/or in the mixing and/or settling steps in the solvent extraction and stripping stages in the circuit, is likely to build, rather than to dissipate. That buildup of static charge in the organic phase could result in a discharge that might result in a fire in the extraction circuit.
One potential solution to an electrostatic charge buildup is the addition to the organic phase of conductivity aids, such as those used in pumped hydrocarbon products, such as kerosene, or motor fuels, such as diesel fuel and jet fuel. Examples of such conductivity aids are the Octastat® products, marketed by Octel Corporation, case studies with which, for Olympic Dam Operations' copper extraction circuit, have been reported in “Additives to Increase the Conductivity of SX Organics to Reduce Static Generation”, M. Giles and D. White, Alta 2006 Copper Conference, May 18-19, Perth, Australia. These Octastat products, which are mixtures of quaternary ammonium compounds with an organic anionic compound, typically a sulfonic acid, unfortunately, may have an adverse effect on the metal extraction process. Their sulfonic acids, which are strong acids, can promote the hydrolysis of the oximes, resulting in degradation of the extractant formulation, and/or they may promote physical problems in the solvent extraction circuits, such as phase separation and/or entrainment issues.
Additives for and/or modifiers of extraction and stripping equilibria are frequently incorporated in commercial solvent extraction reagent formulations, which include the so-called “strong” phenolic oxime extractants. Such extractants are capable of forming very stable complexes/chelates with copper at quite low pH's and, consequently, require the use of very highly-acidic aqueous stripping solutions in order to effect the breakdown of the copper-extractant complex. Where extreme acidity of stripping solutions could generate problems in subsequent conventional electrodeposition processes, modifiers may be incorporated in these solutions in order to shift equilibria in a manner facilitating stripping at lower acidities and to enhance overall metal extraction efficiency.
The use of such equilibrium modifiers (also known as “thermodynamic modifiers”) in combination with oximes, such as the broad range of chemical classes that may be used as equilibrium modifiers in combination with, e.g., 5-nonylsalicylaldoxime (NSO), is described in U.S. Pat. No. 6,231,784 (Virnig et al), the contents of which are incorporated herein by reference. The use of such modifiers in organic solutions for extracting copper salts from aqueous acidic, particularly sulfate, solutions is well known, as is the use of equilibrium modifiers in combination with either or both aldoxime(s) or/and ketoxime(s), such as described in U.S. Pat. No. 6,210,647 B1 (Virnig et al), the contents of which are incorporated herein by reference, in organic solutions for extracting metal salts from aqueous ammoniacal solutions.
Some of these formulations incorporating a modifier include P5100, a mixture of a nonylphenol (NP) modifier in combination with NSO, introduced by ICI to the industry in the early 80's. Henkel subsequently introduced LIX®T 622 extraction reagent formulation, a mixture of isotridecyl alcohol (TDA) with 5-dodecylsalicylaldoxime, and then LIX® 622N extraction reagent formulation, a mixture of TDA in combination with NSO. U.S. Pat. Nos. 4,978,788; 5,176,843; 5,281,336; 6,113,804; and 6,277,300 (all Dalton et al), the contents of which are incorporated herein by reference, describe formulations based on the use of highly-branched alcohols and esters, such as 2,2,4-trimethyl-1,3-pentanediol di-isobutyrate (TXIB), as modifiers. And U.S. Pat. No. 6,177,055 (Virnig et al), the contents of which are incorporated herein by reference, discloses the use of linear esters, such as di-n-butyl adipate (DBA), as equilibrium modifiers for extraction reagents.
The effects of such equilibrium modifiers in extraction formulations have also been reported in “Discussion of the Physiochemical Effects of Modifiers on the Extraction Properties of Hydroxyamines. A Review”, A. M. Sastre and J. Szymanowski, Solvent Extraction and Ion Exchange, Vol. 22(5), pp 737-759 (2004); and in “Equilibrium Modifiers in Copper Solvent Extraction Reagents—Friend or Foe?”, G. Kordosky and M. Virnig, Proceeding of Hydromet 2003, TMS, 2003.
However, kinetic additives and/or equilibrium modifiers have not been reported as having any effect on the conductivity of the organic phase in a solvent extraction circuit. Esters, which are well-known equilibrium modifiers, appear to impart no or only a marginal increase in conductivity to the organic phase of a leaching-solvent extraction-stripping-electrowinning circuit, nor do ethers seem to offer such an advantage, suggesting that there is no direct correlation between equilibrium modification and conductivity enhancement in solvent extraction circuits—certainly not to the extent sufficient to provide a beneficial effect on the conductivity of the organic phase of a metal extraction circuit.